GULF STREAM CLOUD FORMATION DUE TO SEA BREEZE CIRCULATION
Andrew Condon
Department of Marine and Environmental Systems
Florida Institute of Technology
Melbourne, FL 32901
Abstract:
Satellite imagery, surface analysis, and cross sectional analysis are used to determine the
presence of an organized cloud line along the sea surface temperature gradient that
defines the Gulf Stream. This line features similar characteristics to the observed and
well documented sea breeze front. The Gulf Stream cloud line (GSCL) results from a
thermal circulation cell identical in nature to the sea breeze circulation cell that forms due
to the temperature contrast of the ocean waters. Over a three day period in May 2004, the
waters offshore of Cape Canaveral, Florida were monitored for the presence of the
GSCL.
On the first day, May 25, the front was present. The cloud feature is easily seen
on visible satellite images. The second and third day did not show any signs of an
organized GSCL. The existence of the front is backed up with cross sectional analysis of
winds, omega, and relative humidity. The cross sections for day one show the circulation
cell associated with the Gulf Stream as well as the traditional sea breeze cell.
The large scale flow on day one was easterly. Over the course of the three day
period an upper level ridge slid down to the south of the area. With this motion the upper
level wind direction switched to the west and the cloud line was no longer seen.
Although a longer observation period is needed, easterly flow and an abundance of low
and mid level moisture appear to be necessary ingredients for the formation of the GSCL.
A comparison of the synoptic situation on May 25 and June 25 was made. Both
days featured the development of a cloud line along the Gulf Stream sea surface
temperature gradient. The key variable that both days had in common was very low
convective inhibition. With little convective inhibition, slight convergence of surface
winds was enough to generate clouds.
Introduction:
The sea breeze is a mesoscale phenomenon that occurs over coastal regions
during the warm season. As the sun heats the land, a thermal low develops over the hot
land surface, while a thermal high develops over the cooler ocean surface. A sea breeze
develops which blows from the high over the ocean to the low over the land. The cool air
that is advected over the land surface forms the sea breeze front. The thermal circulation
is completed by rising air over the land diverging out back over the ocean aloft (Ahrens,
2003). This general circulation pattern is displayed in figure 1. A similar thermal
circulation can develop over the ocean itself. Off the coast of east central Florida the
Gulf Stream is separated from the peninsula by a pool of colder water. In the same
manner as the sea breeze circulation, a thermal low develops over the warmer Gulf
Stream waters and a thermal high develops over the colder waters of the cold pool. A
thermal circulation develops and a front can be formed along the Gulf Stream (figure 2).
Figure 1 : Typical Sea Breeze Circulation Cell
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Figure 2 : Possible Sea Breeze and Gulf Stream Circulation Cells
Cloud formation over the Gulf Stream has been found to vary diurnally during the
summer and winter (Alliss and Raman, 1995). The time of cloud formation varies with
the height of the cloud and the time of year. Convergence has been found to play a role
in the formation of many low clouds, while advection of high clouds from over the land
surface contributes to the presence of high clouds over the Gulf Stream. During the
winter due to a large gradient in sea surface temperatures (SST), large gradients in
cloudiness have been observed (Alliss and Raman, 1995). During the summer when the
SST gradient is less pronounced, the cloudiness gradient is also lessened. However,
during the summer mesoscale convergence has been found to be an important factor in
cloud formation. The effects of the air-sea interaction on surface and low-level wind
speeds and low-level temperature and moisture distribution over the Gulf Stream have
been documented (Sweet et. al. 1981). The air over the colder regions was found to be
much more stable, while increased sea state and strong winds were found over the
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warmer waters of the Gulf Stream. This could be due to a thermal circulation cell as
discussed above.
Lericos et. al. (2002) conducted an extensive study into lightning data over the
Florida region. Their study reveals that although lightning activity over the peninsula
peaks in the afternoon, activity over the ocean remains fairly constant during the day. In
fact nocturnal lightning is very frequent over the waters of the Gulf Stream due to the
effects of the land breeze. It may be possible that a thermal circulation cell over the
ocean contributes to Gulf Stream convection. Since sea surface temperatures do not vary
as much as land surface temperatures, the circulation cell can be much longer lived.
Their study also looked at the differences in lightning distribution with regards to the
large scale synoptic flow. They found that during southwesterly flow the lightning flash
densities are enhanced, possibly as a result of advection of clouds off the peninsula or
from enhanced front development associated with the Gulf Stream.
The large scale synoptic flow has been determined to have a large effect on the
sea breeze front. In 1962 Estoque found that an offshore wind intensified the sea breeze
front, while an onshore wind weakened the circulation. The offshore winds help to
concentrate the horizontal temperature gradient, intensifying the front. Since that study
others have found that an onshore wind of just a few meters per second can thwart sea
breeze development, while an offshore wind of up to 11 m/s has little effect on the
circulation (Arritt, 1993). The effects of the synoptic flow on the Gulf Stream circulation
cell have not been reported in the literature.
The goal of this project is to determine what is necessary for a front and
associated cloud line to develop along the Gulf Stream. The existence and whereabouts
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of the GSCL is to be determined for a three day period in May 2004. The large scale
synoptic flow as well as the mesoscale conditions will be examined. Conclusions will be
drawn as to what conditions lead to the development of the GSCL and which conditions
stunt its development.
Methods:
Data from May 25, 26, and 27, 2004 were examined in depth along with a brief
review of the GSCL on June 25. The first objective was to identify whether a front
developed over the Gulf Stream waters. To accomplish this, visible satellite data from
the Global Hydrology and Climate Center (GHCC) was used. Using the GOES East
satellite, visible images up to 1 km resolution were obtained over the state of Florida and
the surrounding waters. On these images the sea breeze front shows up very distinctly as
a line of clouds parallel to the shore. The Gulf Stream Front shows up as a line of clouds
along the Gulf Stream. To determine the location of the Gulf Stream MODIS satellite
information was used. MODIS stands for The Moderate Resolution Imaging
Spectroradiometer. It is an optical scanner aboard the NASA satellite Aqua which has a
spatial resolution between 250 meters and one kilometer. The images give a clear view
of the location of the Gulf Stream.
After the satellite data was examined, a preliminary determination of the existence
of the GSCL was made. To further support the conclusions regarding the front, data from
the Melbourne FL office of the National Weather Service (NWS) was used. The
Advanced Regional Prediction System (ARPS) Data Analysis System or ADAS, was
used for hourly analysis of temperature, pressure, winds, cloud parameters, and soil
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moisture flux divergence to help identify the front. These plots are the result of
observations from a variety of sources including metar reports, the KSC network, the
Florida Automated Weather Network, and aircraft reports among many others. The data
has a 4 km resolution and is valuable in determining the frontal location, although the
observations over the Gulf Stream are sparse (NWS, 2004). Radar reflectivity and wind
data was also used. This data was from the NWS Melbourne office NEXRAD WSR-88D
radar. These data were compared for days when formation of a GSCL was observed
versus days were there was no visible front formation. Another tool used to determine
the existence of the GSCL, where cross section analysis using the Rapid Update Cycle
model. The parameters analyzed included pressure vertical velocity (omega), wind
vectors, and relative humidity values in a cross section from Orlando to 120 nautical
miles (nm) offshore.
To determine what conditions favored the formation of a GSCL and what
conditions deterred its formation, surface, 850 mb, and 700 mb data was studied. The
RUC model was used for an analysis of the overall synoptic weather pattern over the
southeastern United States. In addition the RAOB soundings launched from Cape
Canaveral were used to aid in determination of available moisture and convective
potential. Some of this data was obtained from the University of Wyoming meteorology
website.
Results and Discussion:
Day 1: May 25, 2004 Gulf Stream Front Identification
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The sea surface temperatures off the Florida Coast are displayed in figure 3.
From this figure the Gulf Stream current is fairly obvious. It is the area of darker red,
signifying the warmer surface waters of the Gulf Stream. The Gulf Stream flows parallel
to the Florida coastline close to shore until the Jupiter/Palm Beach area. Here it tends to
move offshore a little further as it progresses northward. Off of Cape Canaveral the Gulf
Stream is located well offshore with a pool of colder water between the Cape and the
Gulf Stream.
The surface winds over the peninsula start out blowing out to sea, as is expected
with a land breeze. Over the Atlantic there is a small area of convergence in the vicinity
of the Gulf Stream just east of the Cape as seen in figure 4. Winds in this area are fairly
light and converging. An hour later at 1330 UTC there is still a predominately offshore
flow and convergence in the Gulf Stream region. Figure 5 is a satellite image of the
region. A cloud feature in the vicinity of the Gulf Stream is easily seen at this time. By
1515 UTC the cloud feature is much more distinct as a narrow line of clouds stretching
from the Jupiter area along the Gulf Stream to offshore of Daytona Beach (figure 6). The
ADAS analysis no longer displays as high of values for surface convergence and the
confluence in the wind field is not as obvious but the cloud feature persists.
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Figure 3: Sea Surface Temperature map, showing location of the Gulf Stream in Dark Red
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Figure 4: Surface Moisture Divergence and Surface Winds at 1230 UTC May 25, 2004
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Figure 5: Visible Satellite Image at 1331 UTC May 25, 2004
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Figure 6: Visible Satellite Image at 1515 UTC on May 25, 2004
As the day progresses the line of clouds along the Gulf Stream is still present
although it diminishes in intensity. Figure 7 shows the visible satellite image from 1631
UTC. The line of clouds along the Gulf Stream is still visible and in the same location as
the line at 1515. However this line has diminished in size and continues to do so
throughout the rest of the day. By 1815 a faint line of clouds remains (figure 8). The
southern edge of this line has moved to the west and is almost onshore while the northern
edge has remained nearly stationary, along the Gulf Stream, throughout the day.
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Figure 7: Visible Satellite Image at 1631 May 25, 2004
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Figure 8: Visible Satellite Image at 1815 May 25, 2004
The ADAS analysis shows slight divergence right along the immediate coastal
area. The wind field also shows some diffluence along the western edge of the Gulf
Stream, but the majority of the Gulf Stream is dominated by light northerly winds. As
time progresses the size of the sea breeze cell increases as a larger fetch of wind off the
ocean comes ashore. This increase in the sea breeze circulation pattern may explain the
westward movement of the southern edge of the cloud line. By 1730 the cell reaches far
offshore into the vicinity of the Gulf Stream (figure 9).
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Figure 9: Surface Moisture Flux Divergence and Surface Winds at 1730 May 25, 2004
An analysis of a cross section from Orlando to the NDBC Buoy number 41010
located 120 nautical miles (nm) offshore shows the formation of the cloud feature very
well. The total distance of this cross section is estimated to be around 150 nm. Each tick
mark in the cross section therefore represents around 11.5 nm. The coastline is around 3
marks east of MCO and the Gulf Stream region is between 5 and 10 marks east of MCO.
By 1000 UTC a large area of negative omega values are present in the offshore waters,
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about halfway between Orlando and the buoy (figure 10). As time progresses this area of
rising motion tends to move very slightly to the east. The omega values also tend to
increase throughout the day. By 2100 UTC there is a strong area of rising motion
offshore (figure 11). This diminishes as the rest of the day progresses.
Figure 10: Cross section from Orlando to 120 Miles Offshore, showing Omega and Wind Vectors.
1000 UTC May 25, 2004
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Figure 11: Cross section from Orlando to 120 Miles Offshore, showing Omega and Wind Vectors.
2100 UTC May 25, 2004
To further verify the presence of a front and clouds, a cross section of the relative
humidity values was examined for the same area. The highest relative humidity values,
indicating the air is close to saturation and clouds are likely to develop, are in red in
figures 12 and 13. The first sign of high relative humidity values appears in the 1500
UTC cross section shown in figure 12. At this time a line of clouds was already visible
on the satellite imagery. By 2100 UTC (figure 13), an area of high relative humidity
values in the Gulf Stream vicinity is readily seen. This area corresponds well with the
area of rising motion in figure 11.
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Figure 12: Relative Humidity Cross Section from Orlando to 120 nm Buoy at 1500 UTC May 25,
2004
Figure 13: Relative Humidity Cross Section from Orlando to 120 nm Buoy at 2100 UTC May 25,
2004
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There is conclusive evidence that there was formation of a front similar in
characteristics to the sea breeze front along the Gulf Stream. The ADAS temperature
analysis plots for this day support the scenario illustrated in figure 2. A temperature plot
is shown in figure 14. The cold pocket of surface temperatures is clearly illustrated
between the warmer land of the peninsula and the warm waters of the Gulf Stream.
Figure 14 : Surface Temperatures over Florida and Surrounding Coastal Waters. 1630 UTC May
25, 2004
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Day 2: May 26, 2004 Gulf Stream Front Identification
The second day of observations was a completely different day from the first.
The visible satellite images from day two do not show any cloud feature over the Gulf
Stream at any time during the day (figure 15). The winds offshore do not show any signs
of convergence throughout the day. The overall wind speed is slightly greater and the
wind direction remains westerly over most of the open water with the exception of a
narrow stretch of near shore water (figure 16) due to a narrow sea breeze circulation cell.
Unlike the previous day there is no area of reduced wind speed or area where the wind
direction changes over the Atlantic.
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Figure 15: Visible Satellite Image at 1815 UTC on May 26, 2004
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Figure 16: Surface Moisture Flux Divergence and Winds at 1230 UTC May 26, 2004
The RUC cross sections for May 26 further support the absence of clouds in the
offshore Gulf Stream region. For cross sections from 1000 UTC on the 26th to 0400 UTC
on the 27th there is no consistent large scale region of negative omega values in the
coastal waters. The previous day featured a large well defined region of rising motion,
both over the peninsula and over the Gulf Stream region. On the second day there is a
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large area of positive omega, indicating sinking motion, over the Gulf Stream vicinity for
much of the day as shown in figure 17.
Figure 17: Cross Section from Orlando to 120 miles Offshore showing Omega and Wind Vectors at
1900 UTC May 27, 2004
The relative humidity cross section also shows no sign of cloud development.
Figure 18 is a cross section for the same time period as figure 17. This shows low
relative humidity values that will not support cloud development. This leads to the
conclusion that the sea breeze front was not present on day two.
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Figure 18: Relative Humidity Cross Section from Orlando to 120 nm Buoy at 1900 UTC May 26,
2004
The surface temperature analysis for day 2 does not show any resemblance of a
front in the Gulf Stream vicinity. The temperatures offshore are uniform (figure 19).
Unlike the previous day, the analyzed cold pool, which would allow for formation of the
offshore circulation cell, is not present.
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Figure 19 : Surface Temperature analysis for 1730 UTC May 26, 2004
Day 3: May 27, 2004 Gulf Stream Front Identification
Day three followed a similar pattern to day two. Again there was no visible cloud
line in the vicinity of the Gulf Stream throughout the day. The surface wind analysis for
the day shows a predominately westerly wind throughout the day. As the day progresses
there is an area of slight confluence off the Cape. Rather than westerly and easterly
winds converging like in day one, this confluence is from northwesterly winds meeting
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southwesterly winds as shown in figure 20. This area of confluence does show up in the
satellite image shown in figure 21. However, this cloud feature does not resemble the
line of clouds from day one. It instead is a broader circular area of clouds.
Figure 20: Surface Moisture Flux Divergence and Winds at 1730 UTC on May 27, 2004
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Figure 21: Visible Satellite Image at 1745 UTC on May 27, 2004
The Orlando to 120 mile buoy cross section for the same time period shows an
area of rising motion offshore (figure 22). This is most likely associated with the area of
confluence observed in the wind plot. Also clearly demonstrated in the cross section is
the boundary of the sea breeze circulation cell. Over the land there is strong rising
motion. Offshore there is an area of sinking motion. Further to the east of this area there
is again negative omega values and rising motion. Although there is no line of clouds
along the Gulf Stream, there is some cloud feature that may be associated with the warm
waters of the Gulf Stream.
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Figure 22: Orlando to 120 Mile Buoy, Omega and Wind Vector for 1700 UTC May 27, 2004
The relative humidity cross section for this day seems to support a more
widespread area of clouds (figure 23). The high relative humidity values in the 925 to
850 millibar area may be due to the larger synoptic pattern rather than the mesoscale
thermal circulation.
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Figure 23: Relative Humidity Cross Section from Orlando to 120 nm Buoy at 1700 UTC May 27,
2004
The surface temperature analysis is very similar to the previous days. There is no
cold pool present for most of the day. Offshore surface temperatures remained fairly
uniform throughout the day.
Gulf Stream Front Cloud Formation:
To determine possible causes of the observed cloud formation on the 25th but
none on the 26th and no organized line on the 27th, the synoptic weather pattern was
examined. For all three days a high pressure area dominated over the area. It is this ridge
which played a key role in the upper level wind flow. On day one the ridge axis was
located primarily to the North of the area. The 700 mb heights and winds are shown in
figure 24. The upper level ridge axis runs east to west through the northern portion of
Florida. A similar plot of the heights and winds at 850 mb confirms the presence of a
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ridge of high pressure to the north of the area (figure 25). With the ridge in place to the
north, the upper level flow is from the east. At both levels the figures clearly show a
strong easterly wind. A possible scenario leading to the formation of clouds along the
Gulf Stream would be the predominately easterly flow converging with the westerly flow
from a small circulation cell as shown in figure 26. The easterly flow meeting with the
westerly flow of the thermal circulation would give the necessary convergence in the
Gulf Stream vicinity for cloud formation.
Figure 24 : 700 mb Heights and Winds at 1800 UTC May 25, 2004
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Figure 25 : 850 mb Heights and Wind Vectors at 1800 UTC May 25, 2004
Figure 26 : Possible Cloud Formation Mechanism with Easterly Flow
By examining the sounding for the day further insight into the cloud development
can be found. Figure 27 is the 0900 UTC sounding from Cape Canaveral, the only
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available sounding for the day. There is a lot of low level moisture in the sounding. By
interpolation the convective temperature is only about 73°F. The convective temperature
is the surface temperature that must be reached to start the formation of convective clouds
by solar heating of the surface-air layer (AMS, 2004). From the ADAS surface
temperature analysis (figure14), this temperature was realized throughout the region.
This would help to lead to large scale cloud development. The convergence of the large
scale wind field from the east with the mesoscale thermal circulation wind field from the
west along the Gulf Stream may have lead to the observed line of clouds. The convective
inhibition for this day is 0 J/kg. Convective inhibition is the energy needed to lift a parcel
pseudoadiabatically from its original level to the level of free convection and depends
mostly on mid level temperatures and moisture (AMS, 2004). The larger the negative
value, the less likely clouds will form. For day one the value of 0 indicates that cloud
formation is likely. The fact that the cloud line diminishes in nature during the day is
probably due to changes in the moisture content. If there was an afternoon sounding it
would likely show a decrease in the necessary moisture. This is reflected in the morning
soundings for the 26th.
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Figure 27 : Skew T log P diagram for Cape Canaveral at 0900 UTC May 25, 2004
Day two is again a different picture synoptically. During the course of the day the
upper level ridge starts to slide to the south. The upper level winds start the day from the
east but switch directions as the ridge moves to the south. At 1800 UTC the 700 mb
ridge axis is running across the area as shown in figure 28. This leads to very light upper
level winds in the area. At 850 mb the situation is similar except the ridge axis is a little
further back and light easterly winds are found in the offshore waters. By the end of the
day the ridge has passed to the south and the upper level wind direction has switched to
westerly (figure 29). The changing wind direction and light upper level speeds may have
inhibited cloud development over the coastal waters. With the lighter winds, upper level
divergence would have been reduced, affecting the surface convergence and the
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circulation cell. In order for the thermal cells to develop sufficient upper level
divergence must be present over the warm surface layer.
Figure 28 : 700 mb Heights and Winds at 1800 UTC, May 26 2004
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Figure 29 : 850 mb Heights and Winds at 2000 UTC May 26, 2004
The Cape launched three soundings on this day. The first one launched at 0900
featured a much larger convective inhibition (CINS) value. The value of -303 J/kg
indicates that it is unlikely to have sufficient energy to lift the parcel to its level of free
convection. The 1500 UTC sounding features an even greater CINS value of -606 J/kg.
The final sounding at 2100 UTC has a CINS value of -467 J/kg. The depth of the moist
layer is much shallower on the second day as shown in figure 30. At 0900 the level of
free convection (LFC) for day one was 967 mb. This number is relatively low in the
atmosphere and it is very likely that a parcel will reach this level and continue to rise on
its own. On day two at the same time the LFC was at 803 mb. This further supports the
unlikelihood of cloud development. There was not enough moisture present for large
scale thermal clouds to develop.
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Figure 30 : Skew T log P diagram for Cape Canaveral at 0900 UTC May 26, 2004
By day three the ridge axis was much further to the south. The 700 mb winds
were from the west, southwest (figure 31). These winds coming off the land would be
expected to carry less moisture than the easterly winds off the water of day one. This
likely could have influenced the lack of cloud development along the Gulf Stream. The
wind speeds are greater on day three than day two. At 850 mb the flow is similar to the
flow higher up. It is interesting to note that there is an area of confluence offshore in the
area of the observed cloud feature (figure 32). This confluence seems to be more
associated with the upper level surface ridge than with the mesoscale circulation cells
discussed above. Since this is higher up in the atmosphere the clouds that develop in
association with the confluence may be more readily seen on satellite. This may explain
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why a broad area of clouds was seen in figure 21. There is also the possibility that the
stronger upper level winds advected clouds from over the peninsula to the offshore
waters.
Figure 31 : 700 mb Heights and Winds at 1800 UTC May 27, 2004
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Figure 32 : 850 mb Heights and Winds at 1800 UTC May 27, 2004
The 0900 sounding for the third day features a convective inhibition value of -248
J/kg (figure 33). This falls in between the values for the first and second days. The depth
of the moist layer is very shallow like in the previous day. Most likely this is due to the
winds blowing over the drier air of the peninsula. The estimated convective temperature
is around 83°F. This is slightly greater than the observations of surface temperatures in
the Atlantic. However it is possible that this temperature was reached and cloud
formation would follow.
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Figure 33 : Skew T log P diagram for Cape Canaveral at 0900 UTC May 27, 2004
SST Gradient Cloud Development on June 25:
On June 25, 2004 a line of cumulus clouds along the sea surface temperature
gradient that defines the Gulf Stream was present. Extensive data was not collected on
this day as it was not part of the study, but some conclusions can be drawn from the
limited amount of data that was obtained. A visible satellite image from 1745 UTC
clearly shows the cloud line (figure 34). This line was present for much of the day with
very little movement in any direction. It is interesting to note that the northern portion
remains stationary along the SST gradient (figure 35) while the southern portion remains
stationary, but does not follow the Gulf Stream into shore.
Cross sectional analysis
were not obtained for this day, however it is expected that relative humidity values were
high in the region of the Gulf Stream and that there was strong vertical motion.
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Figure 34 : June 25 1745 UTC Visible Satellite Image
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Figure 35 : June 24 Location of the Gulf Stream
Synoptically, June 25 is similar to late May 25 and early May 26. Once again a
high pressure center is over the area. The 850 mb ridge axis runs nearly exactly over the
region. The overall upper level flow over most of the peninsula is westerly. However the
high pressure center is located off the east coast and upper level winds in this area are
light and variable. The 15:00 UTC RUC 850 mb height and wind analysis is shown in
figure 36. The 850 mb flow at this time is from the west over the peninsula switching to
the south and southeast as it moves offshore. This shows the cyclonic curvature in the
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winds associated with the high pressure area to the east. The winds offshore show signs
of convergence in the flow. The convergence is truly full convergence as the winds
experience speed convergence and directional confluence. This pattern at 850 mb
continues for most of the day as the ridge moves slowly to the south.
Figure 36 : 1500 UTC 850 mb Heights and Winds (University of Wyoming)
The convergence in the wind field is also evident at the surface although the
convergence values may not be all that strong. Much of the convergence appears to be
due to speed convergence. There is some confluence of the surface winds as
southwesterly winds meet with southeasterly winds offshore (figure 37). Overall the
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convergence does not appear to be all that great, but it evidently is enough for cloud
formation.
Figure 37 : 1800 UTC Surface Winds on June 25 (Univeristy of Wyoming)
At 700 mb the upper level flow pattern is somewhat different. At this layer
westerly flow dominates. This westerly flow allows for upper level divergence and may
aid in the formation of lower level cumulus clouds. Generally cloud formation due to
thermal circulation cells is shallow in depth. Most likely the cells do not reach the 700
mb level, so the flow at this level should not inhibit cloud development very much.
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The morning sounding from Cape Canaveral is similar to the sounding on May 25
(figure 38). Once again the CINS value is 0, indicating no convection inhibition. This
seems to be very important in cloud development along the Gulf Stream SST gradient.
The value of 0 is very weak inhibition and can easily be overcome. Since the CINS value
is dependent on mid level temperatures and humidity, the zero value tells us that there
was a good deal of moisture in this level. Although cross sections are not available the
sounding shows a moist surface layer and relatively high dewpoints throughout the
vertical profile. The soundings for both days are similar. The June 25 sounding features
much more moisture throughout the vertical profile however on May 25 there appeared to
be more convergence around the SST gradient. It appears that these two features are
crucial to cloud development along that front.
Figure 38 : 1200 UTC Skew T log P Diagram for Cape Canaveral on June 25
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Conclusion:
A case study of Gulf Stream cloud formation associated with a thermal circulation
cell for a three day period in May 2004 was presented. A variety of meteorological
variables were examined to help identify the presence of an organized cloud line along
the Gulf Stream. On May 25 this line was present. The following two days did not
feature this same cloud formation. Cross sections from May 25 show the existence of a
second thermal circulation cell. This cell exists in the offshore waters. A thermal high
develops in the cold pool between the Florida peninsula and the Gulf Steam. A thermal
low develops over the warmer Gulf Stream waters completing the circulation. This
circulation appears to be dependent on a number of factors.
The large scale synoptic flow was examined for its influence on the development
of the cloud line and circulation cell. It was hypothesized that the easterly flow
experienced on day one was conducive to cloud development. This flow provided
sufficient convergence for cloud formation. The westerly flow that dominated over the
next two days was not conducive to cloud development over the Gulf Stream, although it
may have advected clouds offshore on day three. The amount of moisture in the low and
mid levels was also a very important factor. This fact was illustrated by looking at the
cloud development on June 25. This day featured an abundance of moisture and like
May 25, little convective inhibition. In addition convergence in the area due to the large
scale easterly synoptic flow and the mesoscale westerly flow of the circulation cell
helped cloud formation. This agrees with the conclusions of Estoque back in 1962.
When considering the thermal cell associated with the Gulf Stream, an easterly wind
would be the equivalent of an offshore wind in terms of the sea breeze cell, and would aid
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in formation of the sea breeze or Gulf Stream front. Another important factor is the
existence of the cold pool in the near shore waters. On day one this pool was present, on
day two and three it did not significantly influence the surface temperatures, possibly due
to warm air advection off the peninsula in the westerly flow. In order for the circulation
cell to set up, a temperature gradient must be present.
The key factors in terms of forecasting the GSCL appear to be little to no
convective inhibition, copious low and mid level moisture, a strong temperature gradient
in the sea surface temperatures, and easterly flow leading to convergence along the SST
gradient. When all of these components are present cloud formation can be expected.
References:
C.D. Ahrens. Meteorology Today. 7th Edition. Brooks/Cole – Thomson Learning.
2003.
Alliss, R.J. and S. Raman. Diurnal Variations in Cloud Frequency over the Gulf Stream
Locale. Journal of Applied Meteorology. 34(7): 1578-1596.
------------------------------- Quantitative Estimates of Cloudiness over the Gulf Stream
Locale Using GOES VAS Observations. Journal of Applied Meteorology. 34(7):
500-510.
American Meteorological Society. Glossary of Meteorology.
http://amsglossary.allenpress.com/glossary. June 22, 2004.
R. W. Arritt. Effects of the Large-Scale Flow on Characteristic Features of the Sea
Breeze. Journal of Applied Meteorology. 32(1): 116-125.
M. A. Estoque. The Sea Breeze as a Function of the Prevailing Synoptic Situation.
Journal of Atmospheric Science. 19: 244-250.
T.P. Lericos et. al. Warm Season Lightning Distributions over the Florida Peninsula as
Related to Synoptic Patterns. Weather and Forecasting. 17(1): 83-98.
National Weather Service, Melbourne Florida. Mesoscale Modeling Project at NWS
Melbourne, FL. http://www.srh.noaa.gov/mlb/ldis_index.html. June 15, 2004.
Sweet et. al. Air-Sea Interaction Effects in the Lower Troposphere Across the North
Wall of the Gulf Stream. Monthly Weather Review. 109(5): 1042-1052.
University of Wyoming. Weather. http://weather.uwyo.edu/. June 25, 2004.
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